The endogenous opioid peptides, endorphins and chemically related peptides, produce a variety of effects which suggest their importance as mediators of behavior. Previous studies have provided considerable information about their chemistry and certain biochemical effects and have explored their relationship to pain and analgesia. The endorphins appear to act upon a variety of central nervous system opioid receptors to initiate a variety of behavioral effects. We propose to investigate the receptor interactions of the endorphins and related opioid peptides, their neurotransmitter mediators, and their behavioral roles by thorough characterization of their thermoregulatory effects. Thermoregulatory behavior is relatively simple and well understood in comparison with many other, more complicated, behaviors and lends itself well to the types of studies we propose. The thermoregulatory actions of alpha-, beta-, and gamma-endorphin, met- and leu-enkephalin, ACTH, substance P, neurotensin and vasopressin will be characterized, sensitivity to naloxone antagonism established, and specific tolerance induced. Neurotransmitter antagonists will be characterized, sensitivity to naloxone antagonism established, and specific tolerance induced. Neurotransmitter antagonists will be administered to explore mediators responsible for effects of each opioid peptide. Effects and roles of endorphins in stress-induced hyperthermia, behavioral thermoregulation and temperature stress will be investigated to establish their actions in an uncomplicated behavioral system.